This invention relates to a new process for removing suspended, emulsified and/or colloidally-dissolved solid or liquid impurities from mixtures of water with water-miscible solvents.
Organic solvents diluted with water are formed in the chemical industry in many product areas. For example, in the production of dyes or in the production of synthetic fibers such as elastic polyurethane fibers or polyacrylonitrile fibers which are spun from organic solution, the product dye or fiber contains residual quantities of solvent. This residual solvent is washed out by means of water. Mixtures of water and solvent are thus formed. These mixtures which have a water content of from 75 to 99 weight percent (generally, from 85 to 97 weight percent) not only contain the washed out solvent but also impurities arising from production. In the case of production of polyacrylonitrile fibers, these impurities include plasticizers such as stearyl phosphates, polyalkylene polyethers, other emulsifiers, mineral oils, lubricating oils and silicone oils, fatty alcohols, pigments and/or fiber dust. These impurities are generally present in the water-solvent mixture in suspended, emulsified and/or colloidally-dissolved form. For an economical fiber production, however, it is desirable to recover the solvent present in the mixtures and reuse it in the spinning process. Working-up by distillation of the mixtures containing the impurities is generally very difficult because during distillation the impurities are deposited as crusts on the inside walls of the distillation apparatus and thus hinder or even prevent the transfer of heat. Thus it has so far been necessary (particularly in the case of greatly polluted mixtures of water and solvent) to dispense with working-up by distillation, and to subject these mixtures to the biological purification of waste water. Such measures result not only in the overloading of existing sewage plants but also in a loss of solvent.
Removal of such impurities by filtration has also been unsatisfactory because the use of sand and of mixed media filtration materials and of special cloth filters for removing these impurities has been shown to be unsuitable, even with simultaneous use of flocculents.
The use of polyurethane foam particles for removing suspended solids from liquids, particularly water, described in EP-A No. 00 77 411 is also unsuitable.